Soto-Aquino D, Rosso D, Rinaldi C
Department of Chemical Engineering, University of Puerto Rico, Mayagüez, Mayagüez Puerto Rico.
Phys Rev E Stat Nonlin Soft Matter Phys. 2011 Nov;84(5 Pt 2):056306. doi: 10.1103/PhysRevE.84.056306. Epub 2011 Nov 14.
Ferrofluids are colloidal suspensions of magnetic nanoparticles that exhibit normal liquid behavior in the absence of magnetic fields but respond to imposed magnetic fields by changing their viscosity without loss of fluidity. The response of ferrofluids to constant shear and magnetic fields has received a lot of attention, but the response of ferrofluids to oscillatory shear remains largely unexplored. In the present work we used rotational Brownian dynamics to study the dynamic properties of ferrofluids with thermally blocked nanoparticles under oscillatory shear and constant magnetic fields. Comparisons between simulations and modeling using the ferrohydrodynamics equations were also made. Simulation results show that, for small rotational Péclet number, the in-phase and out-of-phase components of the complex viscosity depend on the magnitude of the magnetic field and frequency of the shear, following a Maxwell-like model with field-dependent viscosity and characteristic time equal to the field-dependent transverse magnetic relaxation time of the nanoparticles. Comparison between simulations and the numerical solution of the ferrohydrodynamic equations shows that the oscillatory rotational magnetoviscosity for an oscillating shear field obtained using the kinetic magnetization relaxation equation quantitatively agrees with simulations for a wide range of Péclet number and Langevin parameter but has quantitative deviations from the simulations at high values of the Langevin parameter. These predictions indicate an apparent elastic character to the rheology of these suspensions, even though we are considering the infinitely dilute limit in which there are negligible particle-particle interactions and, as such, chains do not form. Additionally, an asymptotic analytical solution of the ferrohydrodynamics equations, valid for Pe<<2, was used to demonstrate that the Cox-Merz rule applies for dilute ferrofluids under conditions of small shear rates. At higher shear rates the Cox-Merz rule ceases to apply.
铁磁流体是磁性纳米颗粒的胶体悬浮液，在没有磁场的情况下表现出正常的液体行为，但在施加磁场时会通过改变其粘度而不损失流动性来做出响应。铁磁流体对恒定剪切力和磁场的响应受到了广泛关注，但铁磁流体对振荡剪切力的响应在很大程度上仍未得到探索。在本工作中，我们使用旋转布朗动力学来研究在振荡剪切力和恒定磁场下具有热阻断纳米颗粒的铁磁流体的动态特性。还对使用铁流体动力学方程的模拟和建模进行了比较。模拟结果表明，对于小旋转佩克莱数，复数粘度的同相和异相分量取决于磁场强度和剪切频率，遵循具有场依赖粘度且特征时间等于纳米颗粒场依赖横向磁弛豫时间的麦克斯韦类模型。模拟与铁流体动力学方程数值解的比较表明，使用动力学磁化弛豫方程获得的振荡剪切场的振荡旋转磁粘度在广泛的佩克莱数和朗之万参数范围内与模拟定量一致，但在高朗之万参数值时与模拟存在定量偏差。这些预测表明这些悬浮液的流变学具有明显的弹性特征，尽管我们考虑的是无限稀释极限，其中颗粒间相互作用可忽略不计，因此不会形成链。此外，使用了适用于Pe<<2的铁流体动力学方程的渐近解析解来证明Cox-Merz规则适用于小剪切速率条件下的稀铁磁流体。在较高剪切速率下，Cox-Merz规则不再适用。
